FIB path preference

FIB path preference

Paths are given a preference, lowest value is 'best'. Only paths that are up are up contribute to fprwarding - that's unchanged. What's new is that of the path's that re up only those that have the best preference contribute. A poor man's primary and backup. It's not true primary/backup function because the FIB must converge before the lower preference paths are used.

Change-Id: Ie4453c4a7b1094c6c2b51fe1594b8302103bb68e
Signed-off-by: Neale Ranns <nranns@cisco.com>